1. Field of the Invention
The present invention relates to an image input apparatus for optically obtaining data from a paper document.
2. Description of the Related Art
A typical image input apparatus for optically obtaining data from a paper document includes a photocopier and an image scanner. Data from a paper document is input to the image input apparatus. Light emitted from an illumination lamp is reflected from the paper document, and the reflected light is converted to electrical signals in an image sensor having an imaging device element such as a CCD (Charge Coupled Device).
Patent publication JP-A-10-294827 discloses a typical method of installing such an image sensor in an image input apparatus. In the method, an image sensor is installed in an image input apparatus in a two-step manner. First, an image sensor is secured on a plate-like circuit board. Next, the circuit board is secured on a supporting member, such as a bracket, and then is secured in an image input apparatus with a fastening member such as a screw.
Another patent publication JP-A-64-72655 discloses a method of installing an image sensor in an image input apparatus. In the method, melted adhesive material, such as epoxy-type resin, having a thermally tolerant property, is filled to form a layer of adhesive material between an image sensor and a lens casing for supporting a lens, and the image sensor is secured in the image input apparatus by solidification of the adhesive material.
As disclosed in the patent publication JP-A-10-294827, the internal temperature of an image input apparatus changes depending on heat generated in an image sensor or in other components installed on a circuit board, or depending on changes in ambient temperature. The plate-like circuit board includes a circuit pattern layer and a base layer. When temperature of the circuit board changes the circuit board expands or contracts due to differences in thermal expansion coefficients of different layers. This expansion/contraction leads to a situation that the circuit board becomes bent or warped. In such a situation, the position of the image sensor installed on the circuit board is deviated along the optical axis of the lens. More specifically, when the internal temperature of the apparatus rises due to heat generated in the components installed on the circuit board, the circuit board tends to change its shape in such a manner that the image sensor is displaced in a the opposite direction from the lens. On the contrary, when the internal temperature decreases, the circuit board tends to change its shape in such a manner that the position of the image sensor deviates in a direction toward the lens. If the circuit board changes its shape as described above and the position of the image sensor deviates, light reflected from the paper document cannot be focused onto the image sensor precisely. Accordingly, the quality of input images is degraded.
Next, according to the method disclosed in the patent publication JP-A-64-72655, an image sensor is secured to a lens casing by means of a layer of adhesive material; therefore displacement along the optical axis due to temperature change may be reduced to a level less than that of the configuration disclosed in the patent publication JP-A-10-294827. However, if a rapid hardening type of adhesive material is used, the dimension of the layer drastically changes after hardening. Thus, a tedious optical alignment becomes necessary. If adhesive materials used show less change of their dimensions during a hardening process, longer time is required until the layer becomes solid. Thus productivity is degraded.
Further, according to a method disclosed in the patent publication JP-A-64-72655, the image sensor is fixed with a resin; accordingly it is more difficult to remove the image sensor than in the case of using a fastening member such as a screw. Thus, an image sensor fixed according to this method is less environmentally friendly, since it is difficult to recycle the image sensor.